- individual nucleotides of DNA are made up of three compounds:
- deoxyribose (a sugar)
- phosphate group
- an organic base belongin to one of two different groups:
- single-ring bases - cytosine (C) and thymine (T)
- double-ring bases - adenine (A) and guanine (G)
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- made up of two strands of nucleotides (polynucleotides)
- each strand is joined together by hydrogen bonds formed between bases.
- 1953 the structure was worked out by James Watson and Frances Crick
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Pairing of the bases.
- Organic bases contain nitrogen and are one of two types:
- Double-ring structure (adenine and guanine)
- Single-ring structure (cytosine and thymine)
- Those with a double-ring structure have longer molecules than those with a single-ring structure
- the base pairs must always be make up of one of each type, more specifically;
- Adenine always pairs with thymine by means of 2 hydrogen bonds
- Guanine always pairs with cytosine by means of 3 hydrogen bonds
- This means A is complementary to T and G is complementary to C.
- The ratio of A and T to G and C varies from species to species.
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The double helix.
- The uprights of phosphate and deoxyribose wind around one another to form a double helix.
- For each complete turn of the helix there are ten base pairs.
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Function of DNA.
- DNA: the hereditary material responsible for passing genetic information from cell to cell, generation to generation.
- There are around 3.2 billion base pairs in a typical mammalian cell.
- This means there is almost infinite variety of sequences of bases along the length of a DNA molecule.
- This provides the immense genetic diversity within living organisms.
The DNA molecule is adapted to carry out its functions in a number of ways:
- It is very stable and can pass from generation to generation without change
- Its two strands are joined only by hydrogen bonds, which allow them to separate during DNA replication and protein synthesis.
- It is a large molecule so carries a lot of genetic information.
- The base pairs are within the helical cylinder of the deoxyribose-phosphate "backbone" and so the genetic information, to some extent, is protected from corruption by outside chemical and physical forces.
- DNA depends on the sequence of base pairs that it processes.
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